1. Field of the Invention
The present invention relates to a lithographic apparatus and a method for manufacturing a device.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern can be transferred onto all or part of the substrate (e.g., a glass plate), by imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate.
Instead of a circuit pattern, the patterning device can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can be a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
A flat panel display substrate is typically rectangular in shape. Lithographic apparatus designed to expose a substrate of this type can provide an exposure region that covers a full width of the rectangular substrate, or covers a portion of the width (for example half of the width). The substrate can be scanned underneath the exposure region, while the mask or reticle is synchronously scanned through a beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate then exposure can be completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate can be moved transversely after the first scan, and a further scan is typically performed to expose the remainder of the substrate.
In maskless lithography, there is a continual drive to produce smaller pattern features on a substrate, for example by reducing a size of each individually controllable elements within an array of individually controllable elements. In addition, there is a continual drive to improve contrast attainable by arrays of individually controllable elements used to pattern a radiation beam in a maskless lithography process.
One of the obstacles to improving the attainable contrast is the difficulty in providing arrays of individually controllable elements that can generate a “true black,” e.g., can set individually controllable elements to a state in which substantially no radiation is projected onto the substrate in a corresponding portion of the pattern imaged onto the substrate. This is because, for known arrays of individually controllable elements, in addition to active parts of the array, e.g., elements that may be switched between different states to modulate the beam of radiation, there are passive components that may not be switched between states but are integral to the formation of the array. For example, in an array of tilting mirrors, the passive components may include the hinges about which the mirrors tilt and the space between adjacent mirrors that is necessary to provide clearance between the tilting mirrors. Such passive elements of an array of individually controllable elements result in stray radiation being scattered through the lithography system, such that some radiation is always incident on every part of a substrate, e.g., reducing the attainable contrast. In addition, scattering from the edges of the tilting mirrors further increases the stray radiation, reducing the obtainable contrast even further.
Previously, it has been proposed to reduce these problems by modifying the structure of the array of individually controllable elements. For example, it has previously been proposed to provide the tilting mirrors of an array with a quarter wavelength phase step on a half of each mirror. However, such modifications of the arrays of individually controllable elements increase the cost of manufacturing the arrays of individually controllable elements, increase the likelihood of formation errors within the array of individually controllable elements (in turn further increasing the cost of manufacturing the arrays of individually controllable elements and reducing the number of individually controllable elements that may practically be combined in a single array) and make it more difficult to reduce the size of the individually controllable elements.
Therefore, what is needed is a system and method that allow for modulating a beam of radiation.